1. Field of the Invention
Aspects of the present invention relate to a thin film transistor (TFT), a method of fabricating the same, and an organic light emitting diode (OLED) display device having the TFT.
2. Description of the Related Art
In general, a polycrystalline silicon (poly-Si) layer is widely used as a semiconductor layer of a thin film transistor (TFT), because the poly-Si layer has a high field-effect mobility, can be applied to a high-speed operating circuit, and can be used to configure a complementary-metal-oxide-semiconductor (CMOS) circuit. A TFT including the poly-Si layer is typically used as an active device of an active-matrix liquid crystal display (AMLCD), or a switching or driving device of an organic light emitting diode (OLED).
Methods of crystallizing an amorphous silicon (a-Si) layer into a poly-Si layer may include a solid phase crystallization (SPC) method, an excimer laser crystallization (ELC) method, a metal induced crystallization (MIC) method, and a metal induced lateral crystallization (MILC) method. In the SPC method, an a-Si layer is annealed for several hours, at temperatures below 700° C., at which a glass substrate used in a TFT is transformed. In the ELC method, excimer laser beams are radiated onto an a-Si layer, so that the a-Si layer is heated to a high temperature, in a very short amount of time. In the MIC method, a metal, such as nickel (Ni), palladium (Pd), gold (Au), or aluminum (Al), is brought into contact with, or doped into, an a-Si layer to induce a phase change of the a-Si layer into a poly-Si layer. In the MILC method, a silicide, formed by reaction of the metal with silicon, laterally diffuses, so as to sequentially induce crystallization of an a-Si layer.
However, the SPC method takes too much time and may lead to the deformation of a substrate, because the substrate is annealed at a high temperature, for a long period of time. Also, the ELC method requires an expensive laser apparatuses and results in the formation of protrusions on a poly-Si surface, thereby degrading the interfacial characteristics between a semiconductor layer and a gate insulating layer. Furthermore, when the MIC or MILC method is employed, a large amount of metal catalyst may remain in a crystallized poly-Si layer, thereby increasing the leakage current from a semiconductor layer of a TFT.
A vast amount of research has been conducted on methods of crystallizing an a-Si layer using a metal catalyst, because the a-Si layer can be crystallized at a lower temperature, for a shorter amount of time, than in an SPC method. Typical methods of crystallizing an a-Si layer using a metal catalyst include an MIC method and an MILC method. In these methods, however, the device characteristics of a TFT may be degraded, due to contamination caused by the metal catalyst.
In order to prevent the contamination caused by the metal catalyst, after the a-Si layer is crystallized using the metal catalyst, a gettering process is performed to remove the metal catalyst. A conventional gettering process may be performed using impurities, such as phosphorus (P) or a noble gas, or by forming another a-Si layer on a poly-Si layer. However, the metal catalyst is not removed effectively from the poly-Si layer, so that a leakage current is still large. Furthermore, in the fabrication process of a top-gate TFT, a gettering site is generally far from a channel region of a semiconductor layer, thereby reducing the effectiveness of the gettering.